The atlas of StW 573 and the late emergence of human-like head mobility and brain metabolism

Abstract

Functional morphology of the atlas reflects multiple aspects of an organism's biology. More specifically, its shape indicates patterns of head mobility, while the size of its vascular foramina reflects blood flow to the brain. Anatomy and function of the early hominin atlas, and thus, its evolutionary history, are poorly documented because of a paucity of fossilized material. Meticulous excavation, cleaning and high-resolution micro-CT scanning of the StW 573 ('Little Foot') skull has revealed the most complete early hominin atlas yet found, having been cemented by breccia in its displaced and flipped over position on the cranial base anterolateral to the foramen magnum. Description and landmark-free morphometric analyses of the StW 573 atlas, along with other less complete hominin atlases from Sterkfontein (StW 679) and Hadar (AL 333-83), confirm the presence of an arboreal component in the positional repertoire of Australopithecus. Finally, assessment of the cross-sectional areas of the transverse foramina of the atlas and the left carotid canal in StW 573 further suggests there may have been lower metabolic costs for cerebral tissues in this hominin than have been attributed to extant humans and may support the idea that blood perfusion of these tissues increased over the course of hominin evolution.

StW 573 <b><i>Australopithecus prometheus</i></b>: Its Significance for an Australopith Bauplan

“…Yet it continued to engage extensively in arboreal activity, likely using a mix of (sometimes) hand-assisted bipedality and quadrumanous climbing. Its lower limb was very similar to our own, as far as joint shape was concerned, although longbone length was primitively short, which may have limited endurance in terrestrial walking, and as noted above, the semicircular canals [Beaudet et al, 2019b] and joints in the atlas [Beaudet et al, 2020] appear to be better adapted for mobility in a three-dimensionally complex arboreal environment. Also, the effectiveness of blood perfusion of the brain, as witnessed by the small crosssection of the carotid foraminae of the atlas , does not appear to be adapted for extended activity in open country.…”

“…In addition to information therein provided on taphonomy by Clarke [2019a] and stratigraphy by Bruxelles et al [2019], a detailed inventory of the StW 573 partial skeleton was presented by Clarke [2019a], and a full description of the skull by Clarke and Kuman [2019]. The endocast and inner ear are reported by Beaudet et al [2019a, b], and the atlas was described in a paper published elsewhere by Beaudet et al [2020]. The long limb bones have been reported in Heaton et al [2019] and the pectoral girdle in Carlson et al [2021].…”

Exploring the relationship between soft and hard tissues: The example of vertebral arteries and transverse foramina

“…More particularly, the foramina in the atlas and axis are particularly large as compared to the other vertebrae and to the actual size of the arteries, which may explain the relatively weak correlation in C1. However, given that atlas and axis are frequently recovered from the fossil hominin record (e.g., Lovejoy et al, 1982 ; Gómez‐Olivencia et al, 2007 ; Beaudet et al, 2020 ), investigating the predictive power of such structures was essential. We do acknowledge that uncertainties remain whether the human reference is the most appropriate model for reconstructing fossil hominin soft tissues.…”

“…Given the rising interest in the study of vertebral arteries in fossil hominins (e.g., Beaudet et al, 2020 ), in this study, we statistically explore spatial relationships between the transverse foramina and the vertebral arteries in the cervical segment of the human vertebral column. More specifically, this study aims to (i) measure the cross‐sectional areas of the transverse foramina and the vertebral arteries passing through the cervical vertebrae from C1 to C6 and (ii) to statistically investigate correlations that could be subsequently used to predict the size of the vertebral arteries in fossil hominins.…”

Shape analysis of the StW 578 calotte from Jacovec Cavern, Gauteng (South Africa)

“…We compared StW 578 to extant specimens and Sts 5 by using landmarkfree registration based on smooth and invertible surface deformation. [26][27][28][29][30] This approach has been previously applied to a number of craniodental and postcranial structures, including endocasts 27,29 , enamel-dentine junctions 28,31 , vertebrae 32 and bony labyrinths 33 and comparisons between the landmark-based and landmark-free approaches have revealed the ability of the latter approach for capturing geometric details and for statistical determination of geometric correspondence 34 . Because the StW 578 calotte is partial, instead of using the whole surface to align specimens as in previous studies [27][28][29] , we used four landmarks, i.e.…”